1. Field of the Invention
The present invention relates to semiconductors devices formed from wide band gap materials, and in particular wide band gap light emitting semiconductor devices utilizing tunnel junctions to increase device light emitting efficiency and to increase the device surface robustness to post growth processing steps.
2. Description of the Related Art
Photonic devices such as light emitting diodes (LED's) and quantum well lasers having a single quantum well (SQW) or multiple quantum wells (MQW) have been fabricated from wide band gap materials. These wide band gap materials include but are not limited to gallium nitride (GaN), indium nitride (InN), aluminum nitride (AlN) and combinations thereof. Photonic semiconductor devices have been described in the Physics of Semiconductors, 2nd Ed., SZE, Wiley Interscience, 1981 Chapter 12, pp. 681-742. Wide band gap materials are described in Solid State Electronic Devices, 2nd Ed., Streetman, Prentice Hall, Inc., 1980.
A simple model of a photonic device generally comprises an active region sandwiched between a layer to carry negative charge carriers to the active region and a layer to carry positive carriers to the active region. In the active region the negative carriers and positive carriers can radiatively recombine to generate light. If the active region has a thickness, and a different composition from the two charge carrying layers, a quantum well is formed. The layers on either side of a quantum well are typically called cladding layers. If the active region is a theoretical construct having no thickness, a diode junction is formed. The layers on either side of a junction are typically called confinement layers. For convenience, these layers in either device will be called confinement layers. The confinement layers can extend from the active region to a metal contact layer, or substrate.
Photonic devices made of wide band gap semiconductor materials (e.g. gallium nitride) typically have an n-type confinement layer and a p-type confinement layer confining the active region. One disadvantage of wide band gap p-type confinement layers is that they are more resistive to current flow (i.e. poor electrical conductivity) compared to n-type wide band gap layers. In devices having a p-type layer with good electrical conductivity, a p-type electrical contact can cover a fraction of the surface of the p-type layer. Current flows from the contact throughout the p-type layer, which provides for uniform current injection into the active layer. The contact also covers a small enough area of the p-type layer's that light emitting from the active layer is not blocked by the contact.
In wide band gap p-type layers, however, the resistance to current flow prevents full lateral current spreading throughout the layer from a metal contact with a small footprint. This results in non-uniform current injection into the active layer and reduced efficiency of the overall device.
One way to address the current spreading problem is to use a metal contact that covers most of the surface of the p-type layer. While this does result in effective current spreading, the contact can absorb much of the light emitting from active layer through the p-type layer, reducing the overall efficiency of the device.
Another solution to this problem is to have a multi-layer metal contact on the p-type layer as described U.S. Pat. No. 6,420,736 to Chen, et al. A first layer covers the entire p-type layer and is only a couple of atom layers thick. Chen et al. describes that this layer permits sufficient quantities of light to pass through the metal layer, so that the overall efficiency of the device is acceptable. A second metal layer being thicker, and making more robust contact, covers a fraction of the footprint of the active device. This robust metal layer, however, will be opaque to the light generated by the active region. The absorbing p-type ohmic contact metal that is used in semi-transparent contacts for p-type up devices reduces the brightness generated by the photonic device.
Some metals capable of forming ohmic contacts to wide band gap p-type layers do not effectively adhere to the surface of the p-type layer. This poor adherence can require the deposition of a two-layer semi-transparent window for the p-type metal contact. A first layer of this contact would be deposited directly on the p-type layer to adhere to the semiconductor surface. A second layer of metal is deposited on the first layer to enhance lateral current spreading across the footprint of the active region.
Another characteristic of p-type wide band gap layers is that they are more fragile than n-type wide band gap layers, which can lead to complications in processing wide band gap photonic devices. For example, when the top surface of a wide band gap photonic device is p-type, great care must be taken during all steps of device processing to minimize damage to the exposed surface of the p-type layer. This is especially difficult in photonic devices where many processing takes place after the deposition of the p-type layers.
One approach to surmounting these problems in p-type devices has been to fabricate a tunnel junction within the p-type confining layer between the active region and the metal electrode layer. This junction allows the p-type carriers to be converted into n-type carriers permitting deposition over the p-type layer of an n-type layer. The n-type layer has benefits that include but are not limited to being more robust during processing, having lower resistance than p-type layers and being more efficient at lateral current spreading than p-type layers. This approach has been described by Seong-Ran Jeon et. al., “Lateral Current Spreading in GaN-Based Light-Emitting Diodes Utilizing Tunnel Contact Junctions,” Applied Physics Letters, Volume 78, No. 21, pp. 3265-3267, 21 May 2001. Jeon used a tunnel diode junction in the p-type confinement layer to convert the charge carriers to n-type.
One difficulty with this approach is tunnel diodes in wide band gap materials tend to be inefficient and highly resistive to current flow. Tunnel junctions formed in wide band gap materials have a tunnel width that has a resistance to tunneling. The size of the tunnel width and its resistance to tunneling increases with an increasing band gap width, and decreases with increasing doping at the junction. The band gap of wide band gap materials tends to significantly degrade the performance of the tunnel junction, while the saturation level of dopants in wide band gap materials is such that even degenerate doping does not reduce the resistance to tunneling encountered by the wide band gap material sufficient to make the device efficient.
A tunnel junction contact to a p-type confinement layer in a wide band gap photonic device with a low tunneling resistance is difficult to form, because of the large band gap and doping limitations that prevent the formation of very thin tunnel widths. A thin tunnel junction in a p-type confinement layer would result in a lower resistance to tunneling, higher current flow through the electrode and a more efficient photonic device.